Moon
From Encyclopædia
The Moon is the only natural satellite of the Earth and a unique member of the solar system in several respects. With a radius of 1,738 km (1,080 mi), it is approximately one-quarter of the size of the Earth and 81.3 times less massive. Although the solar system contains both larger and more massive satellites than the Moon, none except Pluto's moon differs so little from its planet in
Mass or size. Indeed, the Earth-Moon and Pluto-Chiron systems constitute veritable double planets.ASTRONOMICAL DATAThe Moon moves around the Earth in an elliptical orbit of small eccentricity, inclined by 5 deg 8' 43
.4 to the plane in which the Earth revolves around the Sun. Its distance from the Earth varies between 356,000 and 407,000 km (221,000 and 253,000 mi) in the course of each month; the average distance is 384,400 km (238,900 mi), less than 1% of the distance to Venus and Mars, even at the time of their closest approach. The lunar globe appears in the sky as a disc of a little over half a degree (31' 5.2) in apparent diameter.The period in which the Moon completes an orbit around the Earth and returns to the same position in the sky--the sidereal month--is 27 days, 7 h, 43 min, and 11.6 sec. Because the Earth is moving in its orbit around the Sun in the same direction as the Moon, the time needed to return to the same phase--the synodic month--is longer: 29 days, 12 h, 44 min, and 2.8 sec. This period is the time interval that, for example, elapses between two successive full moons, a period that was known within a second even in ancient times. The Moon's
average velocity is 1.023 km/sec (0.635 mi/sec), corresponding to a mean angular velocity in the sky of about 33 minutes of arc per hour, a little greater than the apparent diameter of the Moon.In addition to its motion through space, the Moon also rotates about its axis in a period of one sidereal month, so that it keeps approximately the same side toward the Earth at all times. Nonuniformities in its
orbital motion, however, together with the inclination of the orbit to the ecliptic, cause "optical librations" that allow 59% of the entire lunar surface to be seen from the Earth at one time or another. The remaining 41% was hidden until the Soviet LUNA 3 spacecraft photographed the far side in October 1959. It has since been thoroughly mapped.Internal StructureThe
average density of the Moon, calculated from its size and
Mass, is 3.34 g/cu cm (208.51 lb/cu ft). The range of densities of the lunar surface rocks brought back during the APOLLO PROGRAM did not vary greatly from this figure. This means that materials in the Moon's interior probably do not vary much from those on the surface. The
pressure at the center of a more or less uniform Moon would be about 47.1 kilobars. This value is well in excess of the crushing strength of typical lunar rocks. Thus, when the materials from which the Moon was formed began to accrete, the Moon would have formed into a
globe even though the materials have remained solid throughout its
Mass.Seismometers installed on the Moon by Apollo astronauts provided further indications that the lunar interior is rigid, as well as showing that the Moon is seismically much quieter than the Earth. That is, the records of the quakes--whose centers were located 600 to 900 km (375 to 560 mi) below the surface--indicated that both
pressure and shear elastic waves occurred during moonquakes. This would not have been the
case if the waves had to pass through fluid or semifluid layers in the Moon's interior. In addition the quake records implied, through the very long decay time of the disturbances, that the Moon's surface layers must be highly fractured.The rigidity of the lunar interior was further evidenced by the interaction of the Moon with the solar wind, as registered by spacecraft. The data indicate that the Moon exhibits an electrical conductivity consistent with that of silicate rocks cool enough to act as solids. Finally, the virtual absence of a
dipole magnetic field on the Moon, attested to by many spacecraft, discloses that the Moon does not have a metallic core.Chemical CompositionDirect information on the chemical composition of the Moon became available in 1969 with the return of the first Apollo mission. Although the data refer only to the rocks collected on the surface, there is no reason to believe that the composition of the interior of the Moon would be essentially different. By atomic composition, the most abundant element found on the Moon is
oxygen. It composes 60% of the Moon's crust by weight, followed by 16-17%
silicon, 6-10% aluminum, 4-6% calcium, 3-6% magnesium, 2-5%
iron, and 1-2% titanium. All other elements are present in amounts very much smaller than 1% by weight. The elements
oxygen,
silicon, and aluminum are present on the Moon in amounts comparable to their existence in the crust of the Earth.
iron and titanium contents are distinctly enhanced on the Moon, by comparison to Earth, while the alkali
metals?are less abundant, as are
carbon and
nitrogen.Of the compounds formed by these elements, silica constitutes between 40 and 50% of the Moon's crust by weight, compared to 48.5% in the crust of the Earth. Ferrous oxide and calcium oxide constitute 10 to 20% of each. All oxidized compounds appear to be present on the Moon only in their lowest states of oxidation, because they solidified at temperatures between 1,100 and 1,200 deg C (2,000 and 2,200 deg F). The oxide of hydrogen in the form of water is totally absent on the Moon. No trace of water in any form has been found. The only form of hydrogen present on the Moon is that imported by the solar wind, and any water that might be produced by oxidation of this hydrogen would be quickly dissociated by sunlight.Surface FeaturesNaked-eye as well as more detailed telescopic and satellite observations disclose that the lunar surface consists mainly of two different types of terrain. The first is rough, relatively bright, and replete with mountains and occupies more than two-thirds of the visible hemisphere of the Moon and nine-tenths of its far side. The other type is much darker as well as smoother. Terrains of the first type are usually referred to as "continents". Those of the second type are called maria, Latin for "seas." The term
Highlands, sometimes used for continents, is a misnomer in the literal sense, for not all continental ground is elevated. Maria is an even worse misnomer, because the lunar surface is dry.A telescopic inspection of the Moon reveals that both types of ground are replete with formations commonly called craters (see METEORITE CRATER). Their
number is immense, and they range in size from formations such as Mare Imbrium (Sea of Rains) or Mare Orientale (Eastern Sea), which are more than 1,000 km (620 mi) across,
Down to tiny micron pits etched on crystalline rocks brought to the Earth by the Apollo missions. The origin of all such formations is no longer in doubt. They arise directly or indirectly from impacts of celestial bodies ranging from asteroids and comets to interplanetary dust. Because the surface of the Moon is not protected by any
atmosphere, all bodies that happen to be in a collision course with it will impact with cosmic velocities of several km/sec. A particle moving at a relatively slow speed of 3 km/sec (1.9 mi/sec) possesses a kinetic energy equivalent to that released by an explosion of an equal weight of TNT. When such kinetic energy is dissipated on impact, the outcome is a surface scar commonly called a crater. Craters of small or moderate size were excavated in such a way in the rocky layers on the point of impact by removal of material. For those of large size-- approximately 100 km (60 mi) or larger--the amount of heat liberated by impact was sufficient to flood the floor with molten material. Moreover, in the
case of the largest impact formations encountered on the Moon, the excavation of the initial basin appears to have been followed by its lava flooding only after a few hundred million years.Bright rays up to 16 km (10 mi) wide extend outward for hundreds of kilometers from some of the very largest lunar craters. They consist of lighter-colored materials ejected by the force of the impact forming the craters. Over
geologic time such materials will most likely darken and fade into the rest of the Moon's surface. Other notable surface features include ridges and rilles, both of which may also extend for several hundred kilometers. Rilles are narrow lunar valleys, sometimes deep enough to be considered canyons. Sometimes they are fairly straight, but other rilles wander sinuously like riverbeds. Rilles may possibly have been formed in the Moon's earlier years by subsurface flows of molten rock, or lava, leaving hollow tubes into which surface materials eventually collapsed.MineralogyThe dark crystalline materials that fill the basins of lunar maria can be described as gabbroid basalts--materials akin to lavas (see LAVA) known on the Earth but enriched with
iron and titanium. In contrast, the contin?ntal areas of high reflectivity appear to consist of feldspathic rocks similar to terrestrial granites (see
GRANITE), including a nearly pure feldspar called anorthosite. Anorthosites replaced the
iron or magnesium of basaltic rocks with aluminum, making them lighter in weight as well as
color. The very existence of anorthosites on the Moon implies chemical differentiation of the crust, in the course of which heavier elements such as
iron were separated from lighter ingredients. Moreover, anorthosites consist mostly of coarse-grained minerals, which means that they must have cooled off slowly from the melt, and thus not on the lunar surface.
